Steam turbine
]] A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into useful mechanical work. Improvements It has completely replaced the reciprocating piston steam engine (invented by Thomas Newcomen and greatly improved by James Watt) primarily because of its greater thermal efficiency and higher power-to-weight ratio. Also, because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator—it doesn't require a linkage mechanism to convert reciprocating to rotary motion. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency through the use of multiple stages in the expansion of the steam (as opposed to the one stage in the Watt engine), which results in a closer approach to the ideal reversible process. Sizes Steam turbines are made in a variety of sizes ranging from small 1 hp (0.75 kW) units used as mechanical drives for pumps, compressors and other shaft driven equipment, to 2,000,000 hp (1,500,000 kW) turbines used to generate electricity. Types Based on steam Supply and Exhaust Conditions These types include noncondensing, condensing, reheat, extraction and induction. Non condensing Noncondensing or backpressure turbines are most widely used for process steam applications. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. These are commonly found at refineries, pulp and paper plants, and desalination facilities where large amounts of low pressure process steam are required. Condensing Condensing turbines are most commonly found in electrical power plants. These turbines exhaust steam in a partially saturated state, typically of a quality greater than 90%, at a pressure well below atmospheric to a condenser. Steam is extracted at suitable stages of the turbine and sent to boiler feed water heaters to improve overall cycle efficiency. In this case the extraction quantiy is adjusted automatically to suit the requirement of the feed water to the boiler. Reheat Reheat turbines are also used almost exclusively in electrical power plants with capacities generally greater than 200 MW. In a reheat turbine, steam flow exits from a high pressure section of the turbine and is returned back to the boiler where additional superheat is added. The steam then goes back into an intermediate pressure section of the turbine and continues its expansion. Extraction Extracting type turbines are common in process industries. In an extracting type turbine, steam is extracted in considerable quantities from suitable stages of the turbine, and used for industrial process needs. Extraction flows may be controlled with a valve, or left uncontrolled. Induction Induction turbines introduce low pressure steam at an intermediate stage to produce additional power. Casing or Shaft Arrangements These arrangements include single casing, tandem compound and cross compound turbines. Single casing units are the most basic style where a single casing and shaft are coupled to a generator. Tandem compound are used where two or more casings are directly coupled together to drive a single generator. A cross compound turbine arrangement features two or more shafts not in line driving two or more generators that often operate at different speeds. A cross compound turbine is typically used for many large applications. Principle of Operation An ideal steam turbine is considered to be an isentropic process, or constant entropy process, in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine. No steam turbine is truly “isentropic”, however, with typical isentropic efficiencies ranging from 20%-90% based on the application of the turbine. Design The turbine has a casing normally split horizontally, called top and bottom half. Both halves are provided with buckets or reversing blades, suitably mounted. The botttom casing has the bearing housings at the ends for the rotor support and allow the rotor to spin. The rotor has one set of blades suitably mounted. The blades on rotor and casing intermesh with certain minimum clearances. Turbine Efficiency To maximize turbine efficiency, the steam is expanded, generating work, in a number of stages. These stages are characterized by how the energy is extracted from them and are known as impulse or reaction turbines. Most modern steam turbines are a combination of the Impulse and reaction design. Typically, higher pressure sections are impulse type and lower pressure stages are reaction type. Impulse Turbines An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage. Reaction Turbines In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage. Speed Regulation To control the speed of a turbine a governor is essential. Further, turbines need to be run up slowly which warms up the parts gradually, to prevent damage. To day all power station turbines have hydraulic governors with overspeed trips. Uncontrolled acceleration of the turbine rotor can lead to an overspeed trip, which causes the nozzle valves that control the flow of steam to the turbine to close. If this fails then the turbine may continue accelerating until it breaks apart, often spectacularly. However, the turbines used for electric power generation are directly coupled to their generators. As the generators must rotate at constant synchronous speeds according to the frequency of the electric power system, the most common speeds are 3000 r/min for 50 Hz systems, and 3600 r/min for 60 Hz systems. Some large nuclear sets rotate at half those speeds, and have a 4-pole generator rather than the more common 2-pole one. Cost factor Turbines are expensive to make, requiring precision manufacture and special quality materials. A steam turbine is only efficient when operating at its rated specifications. This purchase cost is offset by much lower fuel and maintenance requirements and the small size of a turbine when compared to a reciprocating engine having an equivalent power. Operation and Maintenance When warming up a steam turbine for use the main steam stop valves on the boiler has bypass valves to allow superheated steam to bypass the valves and proceed to heat up the lines in the system along with the steam turbine. Also a turning gear is engaged when there is no steam to the turbine to slowly rotate the turbine to ensure even heating to prevent uneven expansion. When first rotating the turbine by steam the turning gear is disengaged and the astern blades are normally used since they are more robust and not as critical. Problems with turbines are now rare and maintenance requirements are relatively small. Any imbalance of the rotor can lead to vibration, which in extreme cases can lead to a blade letting go and punching straight through the casing. If water gets into the steam (wet steam) and is blasted onto the blades rapid erosion of the blades can occur, possibly leading to imbalance and failure. Also, water entering the blades will likely result in the destruction of the thrust bearing for the turbine shaft. However, the turbines used for electric power generation are directly coupled to their generators. As the generators must rotate at constant synchronous speeds according to the frequency of the electric power system, the most common speeds are 3000 r/min for 50 Hz systems, and 3600 r/min for 60 Hz systems. Some large nuclear sets rotate at half those speeds, and have a 4-pole generator rather than the more common 2-pole one. Uses Electrical power stations use large steam turbines driving electric generators to produce most of the world's electricity. These centralised stations are of different types: fossil fuel power plants,and nuclear power plants. Another use of steam turbines are in ships, where their small size is an advantage. Steam turbine locomotives were also tested, but with limited success. See also Steam_turbine http://en.wikipedia.org/wiki/Steam_turbine The Steam Turbine - all Category:Power stations Category:Energy conversion Category:Turbines